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Arbitrary logic functions | ||
------------------------- | ||
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The `$lut` cell type implements a single-output LUT (lookup table). It | ||
implements an arbitrary logic function with its ``\LUT`` parameter to map input | ||
port ``\A`` to values of ``\Y`` output port values. In psuedocode: ``Y = | ||
\LUT[A]``. ``\A`` has width set by parameter ``\WIDTH`` and ``\Y`` has a width | ||
of 1. Every logic function with a single bit output has a unique `$lut` | ||
representation. | ||
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The `$sop` cell type implements a sum-of-products expression, also known as | ||
disjunctive normal form (DNF). It implements an arbitrary logic function. Its | ||
structure mimics a programmable logic array (PLA). Output port ``\Y`` is the sum | ||
of products of the bits of the input port ``\A`` as defined by parameter | ||
``\TABLE``. ``\A`` is ``\WIDTH`` bits wide. The number of products in the sum is | ||
set by parameter ``\DEPTH``, and each product has two bits for each input bit - | ||
for the presence of the unnegated and negated version of said input bit in the | ||
product. Therefore the ``\TABLE`` parameter holds ``2 * \WIDTH * \DEPTH`` bits. | ||
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For example: | ||
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Let ``\WIDTH`` be 3. We would like to represent ``\Y =~\A[0] + \A[1]~\A[2]``. | ||
There are 2 products to be summed, so ``\DEPTH`` shall be 2. | ||
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.. code-block:: | ||
~A[2]-----+ | ||
A[2]----+| | ||
~A[1]---+|| | ||
A[1]--+||| | ||
~A[0]-+|||| | ||
A[0]+||||| | ||
|||||| product formula | ||
010000 ~\A[0] | ||
001001 \A[1]~\A[2] | ||
So the value of ``\TABLE`` will become ``010000001001``. | ||
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Any logic function with a single bit output can be represented with ``$sop`` but | ||
may have variously minimized or ordered summands represented in the ``\TABLE`` | ||
values. | ||
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.. autocellgroup:: logic | ||
:members: | ||
:source: | ||
:linenos: |